Inactivation of virus in sewage effluent was investigated using a rapid sampling reaction system. The effects of pH and varied reaction conditions are reported. Data presented strongly support the hypothesis that free chlorine is required for significant viral inactivation. The viricidal activity of chlorine at different pH levels correlates better with the persistence of free chlorine than with the theoretical percent HOCl present. It appears that the more significant role of pH in the inactivation of viruses is the control of chlorine wasting side reactions.